1. Field of the Invention
The present invention relates to an image formation apparatus such as copiers and printers, and a development method using such image formation apparatus.
2. Description of the Background Art
A conventional image formation apparatus will be first described.
FIG. 12 is a schematic diagram of a conventional image formation apparatus based on the electrophotographic system. Referring to FIG. 12, the image formation apparatus includes a photoreceptor 1, a charger device 2, an exposure device 3, a developing device 4, a transfer device 6, a cleaner 7, and an optical discharger lamp 8. Photoreceptor 1 is arranged at substantially the center region of the image formation apparatus. Around photoreceptor 1 are disposed charger device 2, exposure device 3, developing device 4, transfer device 6, cleaner 7 and optical discharger lamp 8, sequentially in the direction of rotation of photoreceptor 1.
Exposure device 3 includes a semiconductor laser 301, a polygon mirror 302 for scanning a laser beam, and a lens system 303 to direct the laser beam in a desired shape and scanning speed onto photoreceptor 1 to form an image.
In operation, photoreceptor 1 has its surface charged to a predetermined potential level by charger device 2. Then, a latent image potential corresponding to image information is formed on photoreceptor 1. The electrostatic latent image formed on photoreceptor 1 is conveyed by the rotation of photoreceptor 1 to a development region that faces developing device 4.
In this development region, a development roller is disposed facing photoreceptor 1. The development roller carries a developer precharged to a desired value and having the layer thickness adjusted (referred to as “toner” hereinafter) at its surface. The toner is transferred onto photoreceptor 1 corresponding to the latent image pattern to render the image visible. Following visualization of the latent image potential on photoreceptor 1 by the toner, the toner image is conveyed to a transfer region located at transfer device 6 through the rotation of photoreceptor 1.
A transfer sheet P fed by a sheet feed device not shown is delivered to the transfer region to be synchronously brought into contact with the toner image on photoreceptor 1. A voltage of a polarity of either state corresponding to transferring the toner on photoreceptor 1 to transfer sheet P is applied to transfer device 6, whereby the toner image on photoreceptor 1 is transferred onto transfer sheet P. Transfer sheet P with the toner image is then delivered to have the image fused and fixed on transfer sheet P by a thermal fixing device (not shown). The untransferred toner remaining on photoreceptor 1 after passage of the transfer region is removed from photoreceptor 1 by cleaner 7. A refresh operation of potential is conducted by optical discharger lamp 8 to erase the residual charge of photoreceptor 1. Then, control returns to the initial process.
In the above-described electrophotographic image formation apparatus, characters and the like are binary-recorded by means of the presence/absence of dots based on the toner. In the case of a photographic image or the like, the halftone is expressed by pixels formed of a plurality of binary-recorded dots. If the number of dots in one pixel is increased in order to obtain more gray scale levels that can be represented, the pixel size will become larger. As a result, the resolution defined by the number of pixels is reduced.
To address this issue, various approaches have been employed to obtain more gray scale levels that can be expressed with one pixel without altering the pixel size. For example, the light-on time of the laser beam in the formation of a latent image of one dot is altered to change the size of one dot, or the intensity of the laser beam is altered to change the density of one dot. The technique related to pulse-width modification by altering the light-on time of the laser beam is disclosed in, for example, Japanese Patent Laying-Open No. 3-4244. This publication discloses that the controllability and stability of the gray scale can be improved at the image area of low image density by setting the spot diameter of the laser beam to not more than 0.7 times the dot pitch (63.5 μm for 400 dpi) to increase the contrast of the latent image potential.
There is now a greater demand for higher resolution in the market. For example, the resolution of approximately 1200 dpi is desired so that the area of slanted lines in a character or the like can be easily identified. It is also desired that one dot is formed at approximately 20 μm to improve the graininess of the highlight area.
If the writing pitch with the laser beam is reduced in accordance with higher resolution, the exposure spot must also be reduced. Consider the case of an isolated dot. Even if the exposure spot of laser is made smaller than the writing pitch, or even if the energy distribution thereof is sharp, the potential distribution will become gentle due to the diffusion of charge generated in the photoreceptor after exposure by laser. In other words, it is desirable that the potential at the surface of the photoreceptor corresponding to an isolated dot shown in FIG. 13A exhibits squareness as shown by the solid line in FIG. 13B. However, the potential corresponds to a gentle curve indicated by the chain dotted line due to charge diffusion. The potential distribution corresponding to the potential at the surface of the photoreceptor becomes gentler as the dot pitch becomes smaller. The peak value of potential (=potential difference for developing) becomes lower as indicated by the chain dotted line in FIG. 13B. Thus, an isolated dot can no longer be developed in a digital manner. An isolated dot can be developed only in an analog manner.
This issue will be described with reference to FIGS. 14-17.
FIG. 14 represents the potential distribution at the toner layer face of one dot of a latent image on the photoreceptor, formed by pulse-width modification, in the case where the dot pitch and the exposure spot are relatively large. In FIG. 14, broken line t represents the potential level of development commencement corresponding to the general developing characteristics shown in FIG. 17 whereas broken line s represents the potential level of development saturation corresponding to the same general developing characteristics shown in FIG. 17.
In the case where the dot pitch and the exposure spot are both relatively large, dots in the lower gray scale level will not be developed since the development commence level t is not reached as shown in FIG. 14. In the invention of the aforementioned Japanese Patent Laying-Open No. 3-4244, the laser spot is reduced in order to solve this problem, whereby dots in the lower gray scale level can be developed stably since the development saturation level s is exceeded as shown in FIG. 15.
However, if the dot pitch is reduced to a level so as to allow realization of the resolution of approximately 1200 dpi according to the approach disclosed in Japanese Patent Laying-Open No. 3-4244, the potential distribution of the toner layer face by a latent image will be as shown in FIG. 16 even if the laser spot is reduced. Dots of the upper gray scale level will be developed in the density increasing region (the potential between development commence level t and development saturation level s), not in the density saturation region. In this density increasing region, any slight variation in the potential of the toner layer determined by the potential distribution in the toner layer, the charged amount of toner, or the toner attaching amount of a latent image will cause variation in the dot diameter. It is therefore difficult to realize stable gray scale levels particularly in a low density region.